Photoresist compositions are used in microlithography processes for making miniaturized electronic components such as in the fabrication of computer chips and integrated circuits. Generally, in these processes, a thin coating of film of a photoresist composition is first applied to a substrate material, such as silicon wafers used for making integrated circuits. The coated substrate is then baked to evaporate any solvent in the photoresist composition and to fix the coating onto the substrate. The photoresist coated on the substrate is next subjected to an image-wise exposure to radiation.
The radiation exposure causes a chemical transformation in the exposed areas of the coated surface. Visible light, ultraviolet (UV) light, electron beam and X-ray radiant energy are radiation types commonly used today in microlithographic processes. After this image-wise exposure, the coated substrate is treated with a developer solution to dissolve and remove either the radiation exposed (positive photoresist) or the unexposed areas of the photoresist (negative photoresist).
Positive working photoresists when they are exposed image-wise to radiation have those areas of the photoresist composition exposed to the radiation become more soluble to the developer solution while those areas not exposed remain relatively insoluble to the developer solution. Thus, treatment of an exposed positive-working photoresist with the developer causes removal of the exposed areas of the coating and the formation of a positive image in the photoresist coating. Again, a desired portion of the underlying surface is uncovered.
Negative working photoresists when they are exposed image-wise to radiation, have those areas of the photoresist composition exposed to the radiation become insoluble to the developer solution while those areas not exposed remain relatively soluble to the developer solution. Thus, treatment of a non-exposed negative-working photoresist with the developer causes removal of the unexposed areas of the coating and the formation of a negative image in the photoresist coating. Again, a desired portion of the underlying surface is uncovered.
Photoresist resolution is defined as the smallest feature which the photoresist composition can transfer from the photomask to the substrate with a high degree of image edge acuity after exposure and development. In many leading edge manufacturing applications today, photoresist resolution on the order of less than 100 nm is necessary. In addition, it is almost always desirable that the developed photoresist wall profiles be near vertical relative to the substrate, photoresist images are free of residues, have good depth of focus and the photoresist have good long term and short term stability. Good lithographic properties are important for the photoresist. Such demarcations between developed and undeveloped areas of the photoresist coating translate into accurate pattern transfer of the mask image onto the substrate. This becomes even more critical as the push toward miniaturization reduces the critical dimensions on the devices.
The trend towards the miniaturization of semiconductor devices has led to the use of new photoresists that are sensitive at lower and lower wavelengths of radiation and has also led to the use of sophisticated multilevel systems, such as antireflective coatings, to overcome difficulties associated with such miniaturization.
Photoresists sensitive to short wavelengths, between about 100 nm and about 300 nm, are often used where subhalfmicron geometries are required. Particularly preferred are deep uv photoresists sensitive at below 200 nm, e.g. 193 nm and 157 nm, comprising non-aromatic polymers, a photoacid generator, optionally a dissolution inhibitor, and solvent.
The use of highly absorbing antireflective coatings in photolithography is a useful approach to diminish the problems that result from back reflection of radiation from highly reflective substrates. The bottom antireflective coating is applied on the substrate and then a layer of photoresist is applied on top of the antireflective coating. The photoresist is exposed imagewise and developed. The antireflective coating in the exposed area is then typically dry etched using various etching gases, and the photoresist pattern is thus transferred to the substrate.
In order to further improve the resolution and depth of focus of photoresists, immersion lithography is a technique that has recently been used to extend the resolution limits of deep uv lithography imaging. In the traditional process of dry lithography imaging, air or some other low refractive index gas, lies between the lens and the wafer plane. This abrupt change in refractive index causes rays at the edge of the lens to undergo total internal reflection and not propagate to the wafer (FIG. 1). In immersion lithography a fluid is present between the objective lens and the wafer to enable higher orders of light to participate in image formation at the wafer plane. In this manner the effective numerical aperture of the optical lens (NA) can be increased to greater than 1, where NAwet=nisin θ, where NAwet is the numerical aperture with immersion lithography, ni is refractive index of liquid of immersion and sin θ is the angular aperture of the lens. Increasing the refractive index of the medium between the lens and the photoresist allows for greater resolution power and depth of focus. This in turn gives rise to greater process latitudes in the manufacturing of IC devices. The process of immersion lithography is described in ‘Immersion liquids for lithography in deep ultraviolet’ Switkes et al. Vol. 5040, pages 690-699, Proceedings of SPIE, and incorporated herein by reference. The numerical aperture of the lens has been increasing to values of greater than 1, e.g. up to 1.4 and greater, so that features of less than 50 nm may be resolved successfully.
In addition to increasing the NA of the lens, immersion lithography requires a photoresist with higher refractive index (n) than one for dry lithography, typically of the order of 1.8 and greater at the imaging exposure wavelength. The complex index of refraction (ñ) consists of n, the refractive index which is the phase velocity, and k, the extinction coefficient which is the amount of absorption loss when the electromagnetic wave propagates through the material. Both n and k are dependent on the frequency (wavelength) and are known by the equation, ñ=n−ik. In increasing the NA beyond 1.3, immersion lithography requires the immersion fluid to have a refractive index greater than water and the photoresist to also have higher refractive index than photoresists used in dry lithography to avoid problems with polarization, linearity, variations in critical dimension through pitch, which are diffraction related problems. A higher refractive index is preferred, but material constraints in lens materials and high refractive index fluids have limited the expectation for photoresist refractive index values to near 1.9 at the imaging exposure wavelength. Photoresists used in dry lithography typically have a refractive index of 1.7. Photoresists having a refractive index of 1.9 and greater are being developed for immersion lithography.
A combination of immersion lithography and high refractive index photoresists coated over one or more antireflective coatings provides the imaging of even smaller features with resolution much below 40 nm. However, in order to prevent the deterioration of photoresist image quality from reflection from the substrate, it is desirable to reduce reflectivity to less than 1%. Unlike a single layer bottom antireflective coating, multilayer bottom antireflective coatings are capable of suppressing reflectivity through a wide range of incident angles. The typical trilayer process is an example of a dual layer antireflection scheme which, for example, consists of upper silicon containing bottom antireflective coating and a carbon rich underlayer bottom antireflective coating with the photoresist coated over the upper layer bottom antireflective coating. With multilayer bottom antireflective coatings, reflectivity of less than 1% is achievable in immersion lithography. One general characteristic to multi-layer antireflection coatings is an increase in (k) value from the upper layer to the bottom layer regardless of the elemental composition of the layers. The lower layer of bottom antireflective coating is highly absorbing and the overall reflectivity control is attributed to a combination of absorption predominantly in the lower layer combined with interference effects predominately in the upper layer. With a suitable low absorption coefficient (k) value and matched index of refraction to the photoresist for the upper layer, it also acts as a good light transport layer into the lower absorbing layer. Thus, absorption has to be low in the upper antireflection layer and the refractive index should be equal to or higher than the photoresist for obtaining proper interference effects through a wide range of angles and to maximize transmission from the photoresist into this layer. The higher refractive index in the upper antireflective coating then requires that any antireflective layer beneath it have a similar refractive index value to maximize transmittance into the lower, more absorbing, antireflective layers.
Antireflective coatings that are spin castable are preferred over those that are vapor deposited, since vapor deposited films require expensive vacuum equipment and tend to coat conformally. Increasing the refractive index of a spin-on bottom antireflective coating to match or exceed the refractive index of the photoresist while maintaining other lithographic properties has proved to be difficult, especially in upper layer bottom antireflective coating which have low absorption values. So far, conventional low absorption bottom antireflective coatings have a refractive index of around 1.7 for organic layers and around 1.6 for hybrid silicon containing materials.
In addition to antireflection properties, advanced bottom antireflective coatings also provide enhanced pattern transfer properties to the substrate. In a single layer pattern transfer process, high etch rate bottom antireflective coatings preserve photoresist film thickness so it can act as a thicker mask in transferring the image to the substrate. In multiple layer pattern transfer process, antireflection coatings act as masking layers themselves with low etch rates during etching of the underlying layer, known as hard mask. For example, by inserting a silicon rich antireflective layer beneath the photoresist and another organic antireflective layer beneath the silicon layer the vertical pattern can be amplified through to the substrate to improve the selectivity between elementally different layers as each layer acts as a mask for the next layer. Thus, there is additionally a need for a bottom antireflective coating which has well defined etch characteristics for different types of bottom antireflective coatings. Integrating antireflection properties with hard mask is becoming increasingly useful at many lithography layers.
Therefore, there is a need for a bottom antireflective coating which has the appropriate etch control and is capable of adequately preventing reflection for use in immersion lithography where a high NA lens is used. In immersion lithography, the antireflective film having optimum lithographic parameters of film thickness, absorption, reflectivity, etc. can be achieved by increasing the refractive index to closely match or exceed the refractive index of the photoresist which is coated over the antireflective coating.
The present invention provides for a novel spin-on antireflective coating composition which is coated below a photoresist and is capable of having a refractive index of greater than or equal to 1.8 at the imaging exposure radiation. The increase in refractive index is achieved by adding functionalities to the polymer which increase the refractive index at the exposure wavelength. The novel antireflective coating is used in a process for imaging the photoresist using immersion lithography.